598 DESCRIPTIVE GEOLOGY. 



porous earthy groundmass. Associated with these are gray semi-vitreous 

 vhyoHtes, having a conchoidal fracture, whicli seem to be simply a more 

 suddenly-cooled variety of the tufa. There also occurs here a dark-gray 

 porphyritic rhyolite, having curious spherical balls, about an inch in diam- 

 eter, of the same material, included in the mass, which is made up of distinct 

 crystals of feldspar, quartz, and occasional hornblende and mica, in a com- 

 pact, semi- vitreous groundmass. 



At the extremity of one of the western spurs, toward Susan Creek, 

 is a flow of earthy, flesh-colored trachyte, which contains scattered crystals 

 of sanidin and mica in a micro-crystalline feldspathic groundmass. This 

 rock resembles the European domite. Under the microscope, it is seen to 

 have a light globulitic glassy base, in which is no microscopical biotite, 

 hornblende, augite, or apatite. It contains, however, granular aggregations 

 of red isotrope garnet, an occuiTence never before observed in any 

 trachyte except that from the island of Ischia in Italy. It contains also 

 tridymite, and occasional small blue grains of a hexagonal shape, which 

 are considered by Zirkel as haiiyne.^ 



A tributary to Susan Creek, whicli runs down from the western 

 slopes of the River Range, exposes under these rhyolites and trachytes 

 a bod}" of augite-andesite. On its weathered surfaces, this rock is of 

 a greenish-gray color, but in fresh fracture almost black, having the 

 resinous lustre peculiar to this type of rocks. In its half-glassy ground- 

 mass,, one can distinguish crystals of feldspar and augite, the former 

 being much more distinct on the weathered surfaces. By the aid of 

 the microscope, magnetite grains are also found, but no olivine, and the 

 groundmass is seen to be made up of yellowish-brown glass and the net- 

 work of interwoven microlites peculiar to the augite-andesites. The glass- 

 inclusions in the feldspars and augites are very abundant, and almost 

 replace the original substance of the crystal, as will be seen by reference 

 to Vol. VI, Plate XI, fig. 2, where a thin section of this rock, showing 

 a feldspar crystal included in glass, is illustrated. 



A light-gray brecciated rhyolite, on the eastern slopes of the range, 

 contains many fragments, which resemble the black and green jaspers of 

 the conglomerate. It contains also fragments of rhyolite, while through 



' Microscopical Petrography, 151. 



